In recent years, much attention has been drawn towards the research and development of organic light-emitting devices. Such enormous increase in research interest is highly correlated to the potential application of OLEDs in commercial flat panel displays. With the advantages of low cost, light weight, low operating voltage, high brightness, robustness, color tunability, wide viewing angle, ease of fabrication onto flexible substrates as well as low energy consumption, OLEDs are considered as remarkably attractive candidates for flat panel display technologies.
Typically an OLED contains several layers of semiconductor sandwiched between two electrodes. The cathode is composed of a low work function metal alloy deposited by vacuum evaporation, whereas the anode is a transparent conductor such as indium tin oxide (ITO). Upon the application of a DC voltage, holes injected by the ITO electrode and electrons injected by the metal electrode will recombine to form excitons. Subsequent relaxation of excitons will then result in the generation of electroluminescence (EL).
In order to achieve higher OLED performance, multiple organic semiconductor layers can be incorporated to further separate the two electrodes. There are two main categories of these semiconductor layers, namely vacuum-deposited small molecules and spin-coated polymeric materials. Both fabrication methods have their respective advantages. Vacuum deposition can allow better control over layer thickness and uniformity, while spin coating offers the ease of use and lower production cost [Burrows, P. E.; Forrest, S. R.; Thompson, M. E. Current Opinion in Solid State and Materials Science, 236 (1997)].
In spite of the fact that electroluminescence from organic polymers was initially reported in the 1970s [Kaneto, K.; Yoshino, K.; Koa, K.; Inuishi, Y. Jpn. J. Appl. Phys. 18, 1023 (1974)], it was only after the report on yellow-green electroluminescence from poly(p-phenylenenvinylene) (PPV) that light-emitting polymers and OLEDs have received much attention [Burroughs, J. H.; Bradley, D. D. C.; Brown, A. R.; Marks, N.; Friend, R. H.; Burn, P. L.; Holmes, A. B. Nature 347, 539 (1990)]. Later on, similar studies were reported by using PPV derivatives as the light-emitting polymers [Braun, D.; Heeger, A. J. Appl. Phys. Lett. 58, 1982 (1991)]. Since then a number of new electroluminescent polymers have been investigated for improved properties.
Electroluminescence of organic materials was discovered in anthracene crystals immersed in liquid electrolyte in 1965 [Helfruch, W.; Schneider, W. G. Phys. Rev. Lett. 14, 229 (1965)]. Although lower operating voltages could be achieved by using a thin film of anthracene as well as solid electrodes, very low efficiency of such a single-layer device was encountered. High-performance green electroluminescence from an organic small molecule, aluminum tris(quinolate) (Alq3), was first reported in 1987 [Tang, C. W.; VanSlyke, S. A. Appl. Phys. Lett. 51, 913 (1987)]. A double-layer OLED with high efficiency and low operating voltage was described, in which Alq3 was utilized both as emitting layer and electron transporting layer. Subsequent modifications of the device with triple-layer structure gave better performance with higher efficiency.
For best performance of phosphorescence-based OLEDs, it is desirable for the semiconducting materials to have short lifetimes. One way is to mix singlet and triplet excited states by making use of spin-orbit (L-S) coupling. In the presence of a heavy metal center, the chance of spin-orbit coupling can be greatly enhanced. Hence, the use of heavy metal complexes in OLEDs is preferred over purely organic materials, in which the lowest energy excited state of an organometallic compound is commonly a metal-to-ligand charge transfer (MLCT) triplet state, mixed with the excited singlet state through L-S coupling, so as to obtain higher photoluminescence efficiencies [Baldo, M. A.; Thompson, M. E.; Forrest, S. R. Pure Appl. Chem. 71, 2095 (1999)]. In 1998, Baldo et al. demonstrated a phosphorescence electroluminescence device with high quantum efficiency by using platinum(II) 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphine (PtOEP) as a dye [Baldo, M. A.; O'Brien, D. F.; You, Y.; Shoustikow, A.; Sibley, S.; Thompson, M. E.; Forrest, S. R. Nature 395, 151 (1998)]. A multilayer device in which the emitting layer of Alq3 is doped with PtOEP showed a strong emission at 650 nm attributed to the triplet excitons of PtOEP. Cyclometalated iridium(III) is known to show phosphorescence and is another class of materials used for high efficiency OLEDs. Baldo et al. reported the use of fac-tri(2-phenylpyridine)iridium(III) [Ir(ppy)3] as phosphorescence emitting material which was doped in 4,4′-N,N′-diarbazole-biphenyl (CBP) as a host in an OLED to give high quantum efficiency [Baldo, M. A.; Lamansky, S.; Burrows, P. E.; Thompson, M. E.; Forrest, S. R. Appl. Phys. Lett. 75, 4 (1999)]. In addition, fac-tri-(phenylpyridine)iridium(III) [Ir(ppy)3] was used as phosphorescence sensitizer for high efficiency fluorescent OLED [Baldo, M. A.; Thompson, M. E.; Forrest, S. R. Nature, 403, 750 (2000)]. Using the concept of a phosphorescence emitter with a higher population of excitons, very high efficiency red fluorescence from [2-methyl-6-[2-(2,3,6,7-tetrahydro-1H,5H-benzo[H]quinolizin-9-yl)-ethenyl]-4H-pyran-4-ylidene]propanedinitrile (DCM2) was found in a multilayer OLED composed of Ir(ppy)3 and DCM2 dopant layers. In a sensitization process, energy is transferred from Ir(ppy)3 to DCM2 to give such high efficiency fluorescence.
Apart from the enhancement of the emission efficiency, the ability to bring about a variation in the emission color would be important. Most of the common approaches involve the use of different emission characteristics for color tuning. Examples that employ a single light-emitting material as dopant to generate more than one emission color have been rare. Recent studies have shown that different emission colors from a single emissive dopant could be generated by using phosphorescent material through a change in the direction of the bias or in the dopant concentration. Welter et al. reported the fabrication of a simple OLED consisting of semiconducting polymer PPV and phosphorescent ruthenium polypyridine dopant [Welter, S.; Krunner, K.; Hofstraat, J. W.; De Cola, D. Nature, 421, 54 (2003)]. At forward bias, red emission from the excited state of the phosphorescent ruthenium polypyridine dopant was observed, while the OLED emitted a green emission at reverse bias in that the lowest excited singlet state of PPV was populated. Adanmovich et al. reported the use of a series of phosphorescent platinum(II) [2-(4,6-difluorophenyl)pyridinato-N,C2′] β-diketones as single emissive dopant in OLED [Adamovich, V.; Brooks, J.; Tamayo, A.; Alexander, A. M.; Djurovich, P. R.; D'Andrade, B. W.; Adachi, C.; Forrest, S. R.; Thompson, M. E. New J. Chem. 26, 1171 (2002)]. Both blue emission from monomeric species and orange emission from the aggregates were observed in such OLED and the relative intensity of the orange emission increases as the doping level is increased. As a result, the electroluminescence color can be tuned by changing the dopant concentration with equal intensities of the monomeric and aggregate bands. In both cases, the change of electroluminescence color in OLED can be accomplished upon a variation of the external stimulus or fabrication conditions while keeping the light-emitting material the same.
Even though there has been an increasing interest in electrophosphorescent materials, particularly metal complexes with heavy metal centers, most of the work has been focused on the use of iridium(III), platinum(II) and ruthenium(II), whereas the use of other metal centers have been much less explored. In contrast to the isoelectronic platinum(II) compounds which are known to show rich luminescence properties, very few examples of luminescent gold(III) complexes have been reported, probably due to the presence of low-energy d-d ligand field (LF) states and the electrophilicity of the gold(III) metal center. One way to enhance luminescence of gold(III) complexes is through the introduction of strong σ-donating ligands, which was first demonstrated by Yam et al. in which stable gold(III) aryl compounds were synthesized and found to display interesting photoluminescence properties even at room temperature [Yam, V. W. W.; Choi, S. W. K.; Lai, T. F.; Lee, W. K. J. Chem. Soc., Dalton Trans. 1001 (1993)]. Another interesting donor ligand is the alkynyl group. Although the luminescence properties of gold(I) alkynyls have been extensively studied, the chemistry of gold(III) alkynyls has been essentially ignored, except for a brief report on the synthesis of an alkynylgold(III) compound of 6-benzyl-2,2′-bipyridine in the literature [Cinellu, M. A.; Minghetti, G.; Pinna, M. V.; Stoccoro, S.; Zucca, A.; Manassero, M. J. Chem. Soc. Dalton Trans 2823 (1999)], but their luminescence behaviour has remained totally unexplored. Yam et al. later synthesized a series of bis-cyclometalated alkynylgold(III) compounds using various strong σ-donating alkynyl ligands, and all these compounds were found to exhibit rich luminescence behaviors at both room and low temperatures in various media [Yam, V. W.-W.; Wong, K. M.-C.; Hung, L.-L.; Zhu, N. Angew. Chem. Int. Ed. 44, 3107 (2005); Wong, K. M.-C.; Hung, L.-L.; Lam, W. H.; Zhu, N.; Yam, V. W.-W. J. Am. Chem. Soc. 129, 4350 (2007); Wong, K. M.-C.; Zhu, X.; Hung, L.-L.; Zhu, N.; Yam, V. W.-W.; Kwok, H. S. Chem. Commun. 2906 (2005)]. The present inventors have described herein the design, synthesis and photoluminescence behaviours of luminescent gold(III) compounds with at least one strong σ-donating group, and the use of these compounds as electrophosphorescent material in OLEDs to give strong electroluminescence with high efficiency.